Influenza viruses are one of the most ubiquitous viruses present in the world, affecting both humans and livestock. Influenza results in an economic burden, morbidity and even mortality, which are significant. There are three types of influenza viruses: A, B and C.
The influenza virus is an enveloped virus which consists basically of an internal nucleocapsid or core of RNA associated with nucleoprotein, surrounded by a viral envelope with a lipid bilayer structure and external glycoproteins. The inner layer of the viral envelope is composed predominantly of matrix proteins and the outer layer mostly of host-derived lipid material. Influenza virus comprises two predominant surface antigens, the glycoproteins HA and neuraminidase (NA), which appear as spikes at the surface of the particles. It is these surface proteins, particularly HA, that determine the antigenic specificity of the influenza subtypes.
Virus strains are classified according to host species of origin, geographic site and year of isolation, serial number, and, for influenza A, by serological properties of subtypes of HA and NA. 16 HA subtypes (H1-H16) and nine NA subtypes (N1-N9) have been identified for influenza A viruses (Webster et al. (1992) Microbiol. Rev. 56:152; Fouchier et al. (2005) J. Virol. 79:2814). Viruses of all HA and NA subtypes have been recovered from aquatic birds, but only three HA subtypes (H1, H2, and H3) and two NA subtypes (N1 and N2) have established stable lineages in the human population since 1918. Only one subtype of HA and one of NA are recognised for influenza B viruses.
Influenza A-type viruses evolve and undergo antigenic variability continuously (Wiley and Skehel (1987) Ann. Rev. Biochem. 56:365). A lack of effective proofreading by the viral RNA polymerase leads to a high rate of transcription errors that can result in amino-acid substitutions in surface glycoproteins.
Vaccination plays a critical role in controlling influenza epidemics. Because of the antigenic variability, patients must receive an annual vaccination against the influenza viruses that are predicted on the basis of viral surveillance data to gain immunity against viruses in circulation.
It would of interest to have an influenza vaccine which would not be restricted by inherent strain-specificity, but which would provide a broad protection against a large spectrum of influenza strains. Such a “universal” influenza vaccine could potentially be made by making use of conserved, evolutionarily-stable, epitopes. It has been proposed to make such universal vaccines by using the highly conserved M2 and NP proteins as antigens (Kaiser (2006) Science 312:380). However, these proteins are not abundant on the surface of virions and responses against M2 and NP are much weaker than those induced by standard seasonal vaccines.
HA is abundant on the surface of influenza virions and is the main target of the immune response against the standard vaccines, such as split vaccines. However, the HA molecule is highly variant, and the immune response to HA is predominantly against the more variable regions. Thus, upon influenza vaccination or infection, the immune response is mainly directed at a limited number of continuously evolving, strain-specific, HA antigenic determinants, resulting in very limited cross-reactivity.
Steel et al. (2010, MBio, 1:1) have described an influenza virus vaccine based on the conserved HA stalk domain. However, antibodies against the head domain are more likely to block infection by interfering with virion binding to host cell sialic acid receptors. Thus, it would be of interest to have a “universal” HA antigen which includes the head domain. WO2012082634 describes amino acid substitutions in HA that reduce the responses to dominant hyper-variable epitopes and thus direct responses more to conserved regions of HA.
However, there is still a need for novel influenza antigens which can induce an immune response which provides broad protection against a large spectrum of influenza strains.
Passive immunization with antibodies is another potential way of combating influenza disease. For example, Okuno et al. (1994, J Virol. 68: 517) have described protection against the mouse-adapted A/FM/1/47 strain of influenza A virus in mice by a monoclonal antibody with cross-neutralizing activity among H1 and H2 strains. Similarly, Smirnov et al. (2000, Arch Virol. 145:1733) have described prevention and treatment of bronchopneumonia in mice caused by mouse-adapted variant of avian H5N2 influenza A virus using monoclonal antibody against conserved epitope in the HA stem region.
However, there is still a need for novel anti-HA antibodies which exhibit neutralizing activity against a broad spectrum of influenza strains.